Field of the Invention
This invention relates to stable aqueous emulsions, coating compositions containing the same, the preparation and use thereof, as film formers or as additives with particular reference to their use as thickeners.
Brief Description of the Prior Art
Acrylic latices and their method of preparation involving free radical emulsion polymerization of monomers in waters, in the presence of surfactants, is also known. The free radical emulsion polymerization procedure generally results in high molecular weight polymeric products which are normally in the range of 100,000 to a few million. In this molecular weight range, coatings prepared with the latices display good mechanical properties such as tensile strength and elongation. It has, however, been found that these latices and coatings prepared therefrom leave something to be desired as to properties such as stability of the latices and chemical resistance of their coatings.
These shortcomings are due, at least in part, to the surfactants which are used in the preparation of the latices. In many instances, the surfactants which are water-soluble cause the coating prepared from the latex to be water-sensitive. In some instances, the acrylic latices and their coatings have been found quite unstable, particularly when subjected to mechanical agitation, to heat or cold. Also, some of the prior art latices, particularly those which are of high acid content, do not form good films in that films therefrom are opaque in nature. The opacity of the films is believed to be a manifestation of incompatibility of these latices with other coating ingredients.
Some of the attempts at improving the stability of the latices and have involved variation in the types and/or amounts of surfactants used in the emulsion polymerization. It has, for example, been suggested that the stability of the latices can be improved by adding further amounts of surfactants, particularly when organic initiators are used, or by adding certain stabilizers after completion of the polymerization step. Latices stabilized in this manner are, however, stable only to a limited degree and are vulnerable to external destabilization.